The technique of magnetron sputtering in order to produce a deposit on a substrate is well known. According to the simple principle of magnetron sputtering, a magnetic field is perpendicularly crossed with an electric field so as to cause a cycloidal motion of electrons adjacent to a target. This motion of electrons serves to prolong the mean free paths of electrons, thereby resulting in an increase in the chance to ionize argon gas particles thereby causing the confining of dense plasma adjacent to the target. Such plasma conditions facilitate the sputtering of target material.
Magnetron sputtering apparatus for producing a deposit on a substrate has made progress in various target configurations. However, most target configurations of these prior art devices seem to be suitable for high rate deposition of various materials to be sputtered, but not so efficient for the deposition of magnetic materials.
It is an object of the present invention to provide an improved sputtering apparatus having a multi-rod type target in which the magnetic flux lines can easily pass through the apex region of each rod without any hindrances, so as to obtain a highly uniform deposit.
Another object of the invention is to provide a magnetron sputtering apparatus suitable for use in connection with the deposition of magnetic materials although the apparatus is also useable for the deposition of various metals.
In the prior art target configurations of magnetron sputtering apparatus for use in connection with the deposition of magnetic materials, there are several drawbacks:
In the case of a single rod type target such as the one of Temescal Co. as illustrated in FIG. 5, a protrusion may be produced in the central part of the apex area of the rod as the sputtering proceeds, and a partial absorption of magnetic flux lines may occur at the apex region of the rod. In FIG. 5, the rod type target 112 is attached to the cathode body 110 which also functions as a backing plate. The anode 115 is positioned adjacent to the target. The transverse magnetic fields are produced by the electromagnets 114 which are positioned adjacent to the chamber 113.
As for the multi-slit type planar target of ULVAC CO. (so called G.T. Target) as illustrated in FIG. 4, the magnetic flux lines which are passing through the slits may be fairly decreased by the obstruction of the target pieces made of a magnetic material. In FIG. 4, the planar type target 104 made of a magnetic material having many slits is positioned on the backing plate 103. A yoke 102 with three permanent magnets is attached to the cathode body 101, and the upper side of the magnets is covered by the backing plate.
The merits of the multi-rod type target of the present invention can be discussed from the following technical points of view. It has been well known that the sputtering rate tends to be increased by oblique incidence of positive heavy-ions as opposed to their normal incidence against the target plane. The sputtering surface of a rod lies on the hemispherical shape of its apex including a slight length of adjacent rod side surface, because the magnetic field passes through this apex region. The configuration of this region of the rod seems to be suited for the oblique incidence of argon ions in comparison with a plane target case.
It is well known that when a sputtering surface is increased, the back diffusion of the sputtered particles can also be increased. Such back diffusion effect is detrimental for the formation of good sputtered film. When the surface area of an apex region is developed into a circular plane area, the size of this developed area can be compared with the whole front area of the backing plate. Accordingly a suitable number of rods can be chosen under the condition that the total of each developed area is equal to the whole front area of the common plate. It has been reported that the effect of back diffusion is greatly suppressed in sputtering, if the glow discharge is maintained in argon gas pressure of 10.sup.-3 torr [J.C. Helmer and C.E. Wickersham, J.Vac. Sci. Technol. A4(3), 408(1986)]. From this point of view, it is understand that if the glow discharge is maintained preferably in the above-mentioned gas pressure, the multi-rod type target with a suitable number of rods can be tolerated against the effect of back diffusion.
From early in the history of heavy-ion sputtering, two main sputtering theories, thermal and collisional, have been advocated by investigators from their independent standpoints. As time elapsed, it was gradually recognized that both theories are true for sputtering phenomena. It is thought by the present inventor that the former is dominant in the ion energy range of less than 1 keV and the latter becomes dominant in the ion energy range of higher than around 4 keV. Recently, magnetron sputtering devices based on the most advanced technique in this field have actively prevailed, and the highest sputtering rate can be obtained at the ion energy of 0.7 keV in such devices.
According to the present inventor's publication with regard to the temperature dependence in sputtering, it has been confirmed, by both theory and experiment, that the highest sputtering yield takes place when the target is at room temperature [H.Y. Lee, Radiation Effects Lott. 43,161 (1979)]. This experiment was done in the ion energy of 1 keV with a silver target. A similar result, due to the same kind of experiment, has been earlier reported [T.W. Snouse and M. Bader, Trans. 8th National Symp. Washington (1961) 1, Pergamon Press Inc., New York (1962), P.271]. This experiment was done in the ion energy range of 3 keV with a copper target. Two engineers of Varian Assoc. have also reported that the sputtering yield is inversely proportional to the target temperature [F. Kloss and H. Herte, SCP and Solid State Technol. 10,48(1967)]. The above-mentioned results are keenly related to the theory of thermal spike effect in thermal sputtering. It has been known that the target materials should be highly pure in sputtering. This may be due to the effect of thermal conductivity which is important in thermal sputtering. If the highest sputtering yield in magnetron sputtering at the ion energy of 0.7 keV is caused by the collisional mode, the purity of the target material should have no meaning.
Now, the above-mentioned results are to be referred to the present invention, where the results are recognizable. Since the configuration of the apex region of a rod is fitted for producing thermal spikes with a higher probability than the case of a plane target of a similar area, this effect may be another reason for having a higher sputtering rate in the multi-rod type target than other current targets.